1. Field of the Invention
The present invention relates to a method for removing an oxide film by which an excess oxide film formed on a wafer outer peripheral portion is removed by a vapor-phase etching method at a time of forming the oxide film as a top coat for preventing autodoping during, e.g., growth of an epitaxial layer. More particularly, the present invention relates to a method for removing an oxide film that enables removing an excess oxide film formed on a chamfered surface and an end surface of a wafer, accurately removing an oxide film formed on a wafer lower surface with a desired edge relief width, and greatly reducing a smudge region produced near an oxide film outer edge on the wafer lower surface.
2. Description of the Related Art
In the advanced LSI (Large Scale Integration), a low-resistance semiconductor wafer such as a p, p+, or p++-type silicon wafer adjusted to a predetermined resistivity by adding a dopant such as boron is used as a substrate, and an epitaxial wafer having an epitaxial wafer with higher resistance than the resistivity of the wafer substrate formed thereto, e.g., a p/p+ epitaxial wafer is used on a surface of this wafer substrate.
Although the epitaxial layer is formed in a reaction container set to a high-temperature state, one of problems in manufacture of the epitaxial wafer is a so-called autodoping phenomenon that a dopant such as boron added to a wafer substrate diffuses to the outside from the wafer substrate and the epitaxial layer serving as a device forming region is contaminated during a high-temperature process for forming the epitaxial layer on this wafer substrate.
To solve such a problem in autodoping, an oxide film is generally conventionally formed on a silicon wafer back surface as a top coat that avoids diffusion of a dopant to the outside before forming an epitaxial layer. Although the oxide film is usually formed by, e.g., a CVD (Chemical Vapor Deposition) method, the oxide film may be partially formed on not only a lower surface of a silicon wafer but also an end surface and a chamfered surface of the wafer, and an edge of a wafer upper surface which is an epitaxial layer forming region during this process. When an epitaxial layer is formed in a state that an excess oxide film is formed on the edge of the wafer upper surface, the epitaxial layer and the oxide film come into contact with each other at this position, and a problem that granular Si particles called nodules are generated in this region occurs.
Since the nodules can be a cause that produces particles in a device process, the above-described contact of the epitaxial layer and the oxide film on the edge of the wafer upper surface must be avoided. Therefore, after processing for forming the oxide film on the wafer lower surface is carried out, before the epitaxial layer is formed, processing of removing the excess oxide film formed on the wafer outer peripheral portion is executed. In general, as shown in FIG. 9, the excess oxide film formed on the chamfered surface and the end surface of the wafer is removed, and the oxide film is removed so as to form a region (an edge relief width) where an oxide film 32 is removed from an interface of the chamfered surface and the wafer lower surface toward the inner side of the wafer until a desired interval a is obtained. It is to be noted that the wafer outer peripheral portion means the edge of the wafer upper surface, the chamfered surface, the end surface, and the edge of the lower surface where the excess oxide film is formed on the surface of the silicon wafer.
As a method for removing the excess oxide film on the wafer outer peripheral portion, a chemical method for bringing an etchant having a solvency action on the oxide film into contact with the wafer outer peripheral portion and removing the oxide film or a mechanical method such as a polishing treatment has been conventionally generally adopted. As the method for removing the oxide film based on the mechanical method, there has been disclosed a method for rotating a wafer while pressing nonwoven fabric cloth impregnated with an etchant against a wafer outer peripheral portion, and thereby removing the oxide film on the wafer outer peripheral portion (see, e.g., Patent Document 1). However, according to this method, the etchant drips toward the wafer lower surface side, performing uniform etching in a wafer outer peripheral direction is difficult, and a problem that an interface of a region where the oxide film (a top coat) is formed on the wafer lower surface and a region where the oxide film on the wafer lower surface is removed is apt to become uneven occurs.
Further, there has been also disclosed a method for polishing a silicon wafer characterized in that an oxide film on a chamfered portion of a wafer is removed and the oxide film on an outer peripheral portion of a wafer back surface is polished in such a manner that a thickness of the oxide film is reduced from a position that is at least 2 mm inside of the outermost peripheral portion of the wafer back surface toward the outside (see, e.g., Patent Document 2). This method can prevent particles from adhering to the surface after handling and provide a silicon wafer whose resistivity is not lowered due to autodoping. However, the method for removing an oxide film based on the mechanical method is superior in an oxide film removing accuracy, but the outer peripheral portion of the wafer can be polished in a single-wafer treatment alone, productivity is poor, a configuration of the apparatus becomes big, and a cost disadvantageously increases.
On the other hand, as a method for analyzing a metal impurity that is present on a silicon surface, there has been disclosed a method for analyzing an impurity that adopts a method for removing an oxide film based on a vapor-phase etching method (see, e.g., Patent Document 3). According to this method, an oxide film is formed on a surface on an upper side of a silicon wafer as an analysis target, then the wafer is mounted on a stage installed in a chamber of a vapor-phase etching apparatus with the wafer surface having the oxide film formed thereon as the upper side. Then, for example, vapor containing a hydrofluoric acid and a hydrogen peroxide solution is supplied into the chamber, and the oxide film on the wafer surface is dissolved and removed. Further, droplets are supplied to the wafer surface from which the oxide film has been removed, and an impurity in the recovered droplets is analyzed.
Patent Document 1    Japanese Unexamined Patent Application Publication No. Sho 62-128520 (L. 3 from the bottom of the left lower column to L. 6 of the right lower column in P. 3, FIG. 2)
Patent Document 2    Japanese Unexamined Patent Application Publication No. 2006-186174 (claim 1, paragraph [0010])
Patent Document 3    Japanese Unexamined Patent Application Publication No. 2005-265718
The present inventors turned their attention to applying the vapor-phase etching technology adopted in the analyzing method disclosed in Patent Document 3 to removal of an excess oxide film formed on the wafer outer peripheral portion. However, as a result of conducting various kinds of experiments, the present inventors acquired the following knowledge.
It was revealed that using the vapor-phase etching method enables removing an excess oxide film formed on an upper surface, a chamfered surface, an end surface, or the like of a wafer, but controlling the oxide film formed on a wafer lower surface to a desired edge relief width (an interval a) is difficult. In particular, in case of increasing a removal width of an oxide film outer peripheral edge on the wafer lower surface and manufacturing a silicon wafer with a wide edge relief width, it was confirmed that increasing the removal width of the oxide film outer peripheral edge has a limit even though a treatment time is increased at a time of vapor-phase etching.
Further, it was confirmed that, when a silicon wafer having an oxide film removed from a wafer outer peripheral portion by the vapor-phase etching method was subjected to epitaxial growth, there occurs a drawback that such an oxide film removal uneven region where a thickness of the oxide film varies (which will be referred to as a smudge region 33 hereinafter) as shown in FIG. 10 is extensively formed near an outer peripheral edge of the oxide film formed on the wafer lower surface.
Generation of the smudge region hardly directly affects the quality of the wafer but, when a width of the smudge region increases to some extent, a product is considered as a product with poor appearance and cannot be shipped. In this case, the entire oxide film must be temporarily removed, the oxide film must be again produced, then the vapor-phase etching has to be carried out, and hence a problem such as an increase in manufacturing cost arises. Further, since the smudge region is a region where the oxide film is very thinly formed, nodules may be possibly generated in the smudge region during an epitaxial growth treatment. Furthermore, when a width of the smudge region is large, a dopant in the silicon wafer may possibly diffuse outward through each position at which the smudge region with the thin oxide film is generated. Therefore, at a time of removing the excess oxide film formed on the wafer outer peripheral portion by the vapor-phase etching method, reducing the generation of the smudge region is effective.